Botrytis cinerea is a necrotrophic pathogenic fungus with an exceptionally wide host range comprising at least 235 possible hosts. Because of its wide host range and because it affects economically important parts of the plant B. cinerea is a major problem in many commercially grown crops. Amongst growers, the fungus is commonly referred to as Botrytis. The cultivated tomato (predominantly Lycopersicon esculentum) is also susceptible to infection by Botrytis and the fungus generally affects stem, leaves and fruit of the tomato plant. In heated greenhouses the occurrence of infections by Botrytis on stems is particularly common.
Botrytis actively kills infected cells, causing soft rot, blights, leaf spot, damping-off and stem cancers. Affected leaves become covered with conidiophores and conidia, and subsequently collapse and wither. The fungus will grow from diseased leaves into the stem and produce dry, light brown lesions a few millimeters to several centimeters in length. Lesions may also form at pruning scars on the stem. The stem lesions may also be covered with a gray mold. In severe cases, the infection girdles the stem and kills the plant. Older, senescent tissue of a tomato plant is usually more susceptible to attack by Botrytis than younger tissue.
In order to prevent the development of Botrytis in greenhouse grown tomatoes, the temperature and relative humidity must be closely regulated. It is further important to provide water without wetting the leaves. For field grown plants, good drainage and weed control should be employed. Moreover, the nutrient levels of the plants must be kept high. However, these preventive measures cannot fully avert the occurrence of considerable yield loss in case of infection.
Fungicides are available for controlling Botrytis in both greenhouse and field grown tomatoes. Examples of some fungicides include Dowicide A® and chlorothalonil, which may also be applied to the tomato fruits after harvest. However, Botrytis is known to have developed resistance against several commonly used fungicides. In addition, the use of fungicides is undesired both from an economic and from an environmental perspective. Presently, there is a need for commercial tomato varieties that exhibit resistance to Botrytis. 
Partial resistance to Botrytis has been found in several wild species of Lycopersicon (Egashira et al. 2000; Nicot et al. 2002; Urbasch 1986). These plants however do not produce commercial crop tomatoes.
It is known from WO 02/085105 that L. hirsutum comprises a genetic region on chromosome 10 of the genome that is involved in partial resistance to Botrytis. The introgression of this genetic material into cultivated tomato varieties is believed to be capable of providing for cultivated tomato plants that are partially resistant to Botrytis. 
Thus far, however, breeding programs aimed at providing resistance to Botrytis in tomato have had limited success. The reason for these poor results is at present not clear. For one part, this may be due to insufficient knowledge on the genetic basis and inheritance of Botrytis-resistance. For another part, this may be due to the lack of proper bioassays for assessing Botrytis-resistance levels in tomato plants obtained in breeding programs. The lack of knowledge and methods also complicates the selection of plants among both wild accessions and offspring plants that comprise genes involved in resistance to Botrytis. 
It is an aim of the present invention to improve the success of breeding programs aimed at providing commercial tomato varieties that are resistant to Botrytis. It is a further aim of the present invention to provide for additional and/or improved resistance to Botrytis in commercial tomato varieties. It is yet another aim of the present invention to provide for a method for finding additional wild Lycopersicon accessions as sources of resistance to Botrytis and for finding additional genetic material in the genome of such plants that is involved in resistance of tomato to Botrytis. Such additional sources and additional genetic material may be used to broaden the basis for the production of Botrytis-resistant varieties of cultivated tomato.